Retinas of normal rhesus monkeys were exposed to argon beams using identical doses to those of the clinical treatment. Also, a small amount of blood was injected into the vitreous to simulate the intravitreous neovascularization. Argon laser treatment was applied on the thin blood streaks over the retina. The treated retinas were examined by electron microscopy at various time intervals following the exposure. The cytological appearances of the retinal damage produced by both ruby and argon lasers are almost identical. Damages are localized in the pigment epithelium and photoreceptor cells and the direct change in the blood vessels is extremely slight. However, at a later stage, occlusion of short segments of blood capillaries is noted in the damaged retina, the photoreceptor cells of which have been eliminated by the direct burn. The present observation has revealed that laser effect on the blood vessel is secondary rather than direct. This information is helpful in evaluating the effect of treatment of vascular diseases of human retinas. The experiment with YAG laser has demonstrated the precise site of heat absorption in the pigment epithelium. This information will help future development of this new laser for medical application.